Optoelectronic technology and its applications are constantly expanding, with the result that integrated optics technology can be used with considerable advantage in communication systems. In general, optical signals embody information bits at very high bit rates (over 100 Gb/s in some cases), where such optical signals are often transmitted via single mode optical fiber. When an optical signal arrives at a receiver, the input optical signal is detected and converted into the transmitted information. In the input optical signal, each bit may be represented as an optical pulse having a particular waveform.
For measuring the waveforms of optical pulses used in high bit rate optical communications, it is common practice and desirable to use optical sampling, since it may exhibit higher sensitivity and improved temporal resolution over conventional electrical-based arrangements. Optical sampling systems often use a “probe” pulse signal (also referred to as a “sampling pulse source”, or simply SPS) in combination with a very fast optical sampling gate to provide the desired high temporal resolution measurement. However, even though improving temporal resolution has been the driving force in the development of optical sampling systems, many other aspects come into play and are currently under investigation. Such aspects include, but are not limited to, signal sensitivity and dynamic range, timing and amplitude jitter of the SPS, optical signal bandwidth range, sample synchronizing schemes, signal clock recovery and signal polarization dependence.
In order to make an optical sampling gate independent of the polarization state of the input optical signal waveform, a variety of polarization diversity schemes have been proposed. By using a sampling gate with a two-path configuration such that the input optical signal is divided into two separate optical paths, it is possible to achieve a significant degree of polarization independence. In one arrangement, a sum-frequency generation technique using a pair of KTP crystals has been reported to achieve a marginal polarization dependence on the order of 0.15 dB, as disclosed in a conference proceeding entitled “Polarisation independent optical sampling system using two-path configuration ”, N. Yamada et al., OFC Proceedings 2004, Atlanta, Ga., paper MF92. In a reference entitled “Polarisation independent optical sampling using four-wave mixing” by A. Tersigni et al. presented at CLEO 2003, Baltimore, Md. as Paper CMR2, an arrangement including a polarization beam splitter and counterpropagating sampling pulses in a highly-nonlinear fiber (HNLF) has achieved a polarization dependence on the order of 0.7 dB. While these results are promising, the various known arrangements all require the use of a two-path configuration, adding both cost and complexity to the sampling system.
Thus, a need remains in the art for addressing the problem of polarization dependence in an optical sampling arrangement without requiring a multi-path solution, with its associated cost and complexity problems.